The Gazette was given access to take photos and video 40ft deep on the bed of the River Tees - where a £100,000 maintenance project is being carried out.

The four huge “fish belly” gates control the flow of the river and each play a vital role in preventing the area from flooding.

At the time of its construction 20 years ago, the barrage was considered the largest civil engineering project in the UK.

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Workers are currently completing their annual inspection of the site where checks are being made on the gate seals, walls, floor and pivot points.

The steel work is also being checked for cracks and damages and crustaceans removed.

As part of the repairs, the Canal & River Trust is holding a special open day giving the public the chance to explore around the structure and delve down into one of the drained gate wells and control tower to see the inner workings up close.

Operations officer Craig Bateman said: “We invited the public to come and have a look last year and it was extremely popular. It’s an opportunity to show locals exactly what we do, how we do it and where money is spent.

“One of our biggest worries would be if the pivots which anchor the gate to the river bed needed replacing. We would have to remove the full gate and re-engineer it, which would take months.”

Lee Butler, team leader at the Tees Barrage says: “We do this work to keep the Tees Barrage in tip top condition. This is an exciting opportunity for us to give people a rare behind the scenes look at our maintenance work and walk onto the Tees’ river bed as well as learn about fish migration, get rare access into the control tower and discover more about the famous white water course.”

The event is between 10-4pm on Saturday, August 22. No pre-booking is required and visitors will join tours on a first come, first served basis. Further details are available at www.canalrivertrust.org.uk/summer-open-days .